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Mirror modifier does not Custom Split Normal Data
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System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GT 1030/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 385.12

Blender Version
Broken: version: 2.80 (sub 58), branch: master, commit date: 2019-04-23 12:31, hash: rB5a144c797a3b

Short description of error
When using Custom Split Normal Data (or just custom vertex normal) with mirror modifier, normal vectors are not mirrored properly.

Exact steps for others to reproduce the error

  1. In default Cube, Object Data, Normals, enable Auto Smooth
  2. Move base cube in edit mode off the origin center
  3. Select all and go to Mesh, Normals, Set From Faces
  4. Select top face and repeat Set From Faces
  5. Add mirror modifier to the object

Results are different, not mirrored normals.

[Based on the attached .blend file (as simple as possible)]



Event Timeline

Jacob Max Fober (Fubax) renamed this task from Mirror modifier messes up Custom Split Normal Data to Mirror modifier does not Custom Split Normal Data.Apr 23 2019, 11:01 PM

quoting @Bastien Montagne (mont29) here:

Thanks for the report, but there is nothing new here, custom normals have never been supported with topology-editing modifiers or operations (like mirror one). Iirc, 2017 GSoC tackling editing of custom normals also added their support to mirror modifier, not sure when we'll have time to merge it in master though :|
But in general, you should never modify topology of your mesh once you have added custom normals, we might end up supporting a limited set of cases (like the mirror modifier), but this is not a good idea anyway. Custom normals are designed to support alterations of the shape (deformations), but absolutely not of the topology, this is not possible. So this is a known limitation/low priority TODO at best. ;)

So will close this as duplicate of T53763